This invention relates to a zinc coating composition and a method of providing zinc with a protective coating. More particularly, the present invention provides a zinc coating composition having as its essential ingredient the isopoly and heteropolymolybdate acids and alkali metal and ammonium salts thereof at a pH that ranges from 1.0 to 5.0. The isopolymolybdate acids consist of clusters of seven or eight molybdenum atoms and associated hydrogen and oxygen atoms. The heteropolymolybdate acids consist of a cluster of seven or eight molybdenum atoms, hydrogen, oxygen and one or two atoms of another element.
Untreated zinc metal quickly develops a white film of zinc oxide or hydroxide. These corrosion products will cause many deleterious effects. For example, zinc oxide prevents paint from adhering to the metal. In addition, the oxide accelerates further corrosion of the metal.
Passivating the metal prevents the formation of zinc oxide or hydroxide (see British patent No. 592,073; Wendorff, Z., Zolnierowicz, A.; Ochronaprzed Korozja, 13,1 (1970); Ostrander, G. W.; Plating, 38,1033 (1951); and British patent No. 594,699). Typical passivating solutions utilize a dichromate or chromate composition. The composition is generally applied to the metal via immersion (see Fishlock, D. J.; xe2x80x9cProduct Finishingxe2x80x9d, 12, 87 (1959). Increasing immersion times up to 300 seconds will generally produce a more effective coating. Immersion times beyond 300 seconds typically do not produce more effective coatings. An untreated surface will show signs of corrosion after 0.5 hours of exposure to a neutral salt spray according to ASTM specification xe2x80x9cB 117xe2x80x9d and a thin chromate film produced by a dip procedure will show signs of corrosion after 12 to 24 hours of salt spray exposure (see; ASTM-xe2x80x9cB201xe2x80x9d).
Reliance on hexavalent chromium has many drawbacks. Hexavalent chromium is extremely toxic and as such more costly to work with. For instance, hexavalent chromium will require special disposal procedures. I have a unique chromium-free zinc coating composition which is superior, in part, because it does not have the toxicity and cost associated with hexavalent chromium. My zinc coating composition utilizes isopoly and/or heteropolymolybdates as its essential ingredients.
The present invention concerns a method for the coating of zinc or zinc coated articles with a composition containing isopolymolybdate or heteropolymolybdate acids, or a mixture of them, at a pH of about 5.0 to 1.0. This invention also concerns a passified zinc or zinc coated article having coated thereon a chromium-free polymolybdate acid protective coating.
Another aspect of the invention is directed at a chromium-free polymolybdate coating composition for coating zinc or zinc plated articles.
Still another aspect of this invention is directed at a manufacture of an article of zinc or zinc coated material having a polymolybdate protective coating thereon.